arXiv subjects

S. Boula

Publications and source records attributed to S. Boula.

8 recordsLinked to original sources

arXiv2026

A self-consistent explanation of the MeV line in GRB 221009A unveils a dense circum-stellar medium

GRB~221009A has been the brightest gamma-ray burst (GRB) observed to date, and its afterglow has been characterized with unprecedented detail at TeV energies by LHAASO. Quite puzzlingly, it is also the most energetic GRB known. Among the riddles posed by this mysterious source, however, the sheer energetics are hardly the most intriguing: an unprecedented emission line at around 10 MeV has been uncovered by a detailed spectral analysis of Fermi/GBM data immediately following the brightest peak in the GRB prompt emission and the peak of the TeV afterglow. The temporal evolution of the line properties can be explained as high-latitude emission from a geometrically thin, relativistically expanding shell where annihilation of a large number of electron-positron pairs took place. We show that this interpretation yields stringent constraints on the properties of such shell, that point to a process that happens at radii typical of external shocks. We then demonstrate that the shell could have been the blastwave associated with the GRB precursor, with the line arising after pair loading of such blastwave as it was illuminated by the bright and hard radiation of the GRB. The scenario, which also explains the abrupt initial rise of the LHAASO afterglow, requires the progenitor of the GRB to have been surrounded by a circum-stellar medium (CSM) extending out to a few 10^15 cm, with a density 10^8-10^9 cm-3 reminiscent of those found in Type IIn supernovae. The consequences of such a CSM on the dynamics and emission of the external shock are yet to be fully explored. If future, more detailed work will confirm the compatibility of the GRB 221009A afterglow with our scenario, this will provide a precious clue to the nature of the progenitor of this peculiar GRB, which could also be present in other bursts that feature a long quiescence followed by a bright emission episode with a hard spectrum.

astro-ph.HE↗

arXiv2025

The SST-1M stereoscopic system

The Single-Mirror Small-Size Telescope (SST-1M) is an Imaging Atmospheric Cherenkov Telescope designed for detecting very high-energy gamma rays. With a compact design achieved through the adoption of silicon-photomultiplier pixels and a lightweight structure, SST-1M offers a large field of view of about 9{\deg} and features a mirror system of 4 m diameter with an optical PSF (at 80% of photon inclusion) of 0.08{\deg} on axis and 0.21{\deg} at 4{\deg} off-axis, and a fully digitizing readout almost deadtime free up to few kHz. The SST-1M achieved a high-performance and cost-effective solution for implementing an array of small-sized telescopes. The stereoscopic system of two SST-1Ms is temporarily installed at the Ond\v{r}ejov Observatory in the Czech Republic. From an altitude of only about 510 m and in harsh meteorological conditions, the system is detecting galactic sources and flares of AGNs. The accurate calibration of the detector and the simulation benchmark are ongoing. The results of its performance are shown. A future final location is being considered and a future performance outlook is discussed.

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arXiv2025

Observation of the Crab Nebula with the Single-Mirror Small-Size Telescope stereoscopic system at low altitude

The Single-Mirror Small-Size Telescope (SST-1M) stereoscopic system is composed of two Imaging Atmospheric Cherenkov Telescopes (IACTs) designed for optimal performance for gamma-ray astronomy in the multi-TeV energy range. It features a 4-meter-diameter tessellated mirror dish and an innovative SiPM-based camera. Its optical system features a 4-m diameter spherical mirror dish based on the Davies-Cotton design, maintaining a good image quality over a large FoV while minimizing optical aberrations. In 2022, two SST-1M telescopes were installed at the Ond\v{r}ejov Observatory, Czech Republic, at an altitude of 510 meters above sea level, and have been collecting data for commissioning and astronomical observations since then. We present the first SST-1M observations of the Crab Nebula, conducted between September 2023 and March 2024 in both mono and stereoscopic modes. During this observation period, 46 hours for the SST-1M-1 and 52 hours for the SST-1M-2 were collected for which 33 hours are in stereoscopic mode. We use the Crab Nebula observation to validate the expected performance of the instrument, as evaluated by Monte Carlo simulations carefully tuned to account for instrumental and atmospheric effects. We determined that the energy threshold at the analysis level for the zenith angles below $30^\circ$ is 1 TeV for mono mode and 1.3 TeV for stereo mode. The energy and angular resolutions are approximately 20% and $0.18^\circ$ for mono mode and 10% and $0.10^\circ$ for stereo mode, respectively. We present the off-axis performance of the instrument and a detailed study of systematic uncertainties. The results of a full simulation of the telescope and its camera is compared to the data for the first time, allowing a deep understanding of the SST-1M array performance.

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arXiv2025

Do multifrequency polarimetric observations of BL Lac rule out a hadronic origin for its X-ray emission?

Recent multifrequency polarimetric observations of the eponymous blazar BL Lac reveal an extremely large degree of polarization in the optical band (average of $25\%$, reaching $45\%$), together with a small ($\lesssim 7\%$) degree of polarization in the X-ray band. This has been interpreted as evidence that the X-rays are produced through inverse Compton emission by relativistic electrons, thus ruling out alternative models based on hadronic processes. Here we revisit the observational evidence, interpreting it in a framework where the observed radiation is entirely produced through synchrotron emission. Electrons produce the radio-to-optical component and protons produce the X-rays and the gamma-rays. We determine the jet magnetic fields from an MHD model of magnetically dominated stationary axisymmetric outflows, and show that the X-ray emission from the protons is naturally less polarized than the optical emission from the electrons. The model parameters required to reproduce the multifrequency polarimetric observations are fully compatible with blazar jets.

astro-ph.HE↗

arXiv2025

How do recollimation-induced instabilities shape the propagation of hydrodynamic relativistic jets?

Recollimation is a phenomenon of particular importance in the dynamic evolution of jets and in the emission of high-energy radiation. Additionally, the full comprehension of this phenomenon provides insights into fundamental properties of jets in the vicinity of the Active Galactic Nucleus (AGN). Three-dimensional (magneto-)hydrodynamic simulations revealed that the jet conditions at recollimation favor the growth of strong instabilities, challenging the traditional view-supported from two-dimensional simulations-of confined jets undergoing a series of recollimation and reflection shocks. To investigate the stability of relativistic jets in AGNs at recollimation sites, we perform a set of long duration three-dimensional relativistic hydrodynamic simulations with the state-of-the-art PLUTO code, to focus on the development of hydrodynamical instabilities. We explore the non-linear growth of the instabilities and their effects on the physical jet properties as a function of the initial jet parameters: jet Lorentz factor, temperature, opening angle and jet-environment density-contrast. The parameter space is designed to describe low-power, weakly magnetized jets at small distances from the core (around the parsec scale). All collimating jets we simulated develop instabilities. Recollimation instabilities decelerate the jet, heat it, entrain external material, and move the recollimation point to shorter distances from the core. This is true for both conical and cylindrical jets. The instabilities, that are first triggered by the centrifugal instability, appear to be less disruptive in the case of narrower, denser, more relativistic, and warmer jets. These results provide valuable insights into the complex processes governing AGN jets and could be used to model the properties of low-power, weakly magnetized jetted AGNs.

astro-ph.HE↗

arXiv2025

Secondary black hole-induced magnetic reconnection in OJ 287: Implications for X-ray and radio emission

OJ 287, a nearby blazar, has exhibited remarkable variability in its optical light curve since 1888, characterized by ~12-year quasi-periodic outbursts. These events are attributed to the orbital dynamics of a supermassive binary black hole system at the heart of the blazar. This study explores the role of magnetic reconnection and the formation of plasmoid chains in driving the energetic processes responsible for OJ 287's variability. We propose that the passage of the secondary black hole through the magnetic field of the primary black hole's accretion disk triggers magnetic reconnection, which contributes to the observed X-ray and radio emission features in OJ 287. We explore the connection between binary black hole interactions, accretion disk dynamics, and the formation of plasmoid chains as the secondary black hole passes through the magnetic field forest from the accretion disk and the jet of the primary. Our approach relies on numerical simulations to understand the formation of plasmoid chains resulting from black hole interactions and accretion disk dynamics. Based on such results, we employ simulation outcomes to examine the potential contribution to observed emissions, validating our assumptions about plasmoid chain creation. With this idea, we aim to establish a direct link between numerical simulations and observed emission, particularly in the case of OJ 287. Our findings confirm that the formation of plasmoid chains coincides with specific anomalous emission events observed in OJ 287. Notably, the radio emission patterns cannot be explained by a single blob model, as the necessary size to mitigate synchrotron self-absorption would be too large. This highlights the complexity of the emission processes and suggests that plasmoid chains could contribute to additional emission components beyond the steady jet.

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arXiv2024

Analysis of commissioning data from SST-1M : A Prototype of Single-Mirror Small Size Telescope

SST-1M is a prototype of a single-mirror Small Size Telescope developed by a consortium of institutes from Poland, Switzerland and the Czech Republic. With a wide field of view of 9 degrees, SST-1Ms are designed to detect gamma-rays in the energy range between 1 and 300 TeV. The design of the SST-1M follows the Davies-Cotton concept, with a 9.42m2 multi-segment mirror. SST-1M is equipped with DigiCam camera, which features a fully digital readout and trigger system using 250 MHz ADC, and a compact Photo-Detector Plane (PDP) composed of 1296 pixels, each made of a hexagonal light guide coupled to silicone photomultipliers (SiPM). Two SST-1M telescopes are currently being commissioned at the Ondrejov Observatory in the Czech Republic, where they are successfully observing Cerenkov events in stereo. This contribution will present an overview of calibration strategies and performance evaluation based on data collected at the observatory.

astro-ph.IM↗

arXiv2023

Mono and stereo performance of the two SST-1M telescope prototypes

The Single-Mirror Small-Sized Telescope, or SST-1M, was originally developed as a prototype of a small-sized telescope for CTA, designed to form an array for observations of gamma-ray-induced atmospheric showers for energies above 3 TeV. A pair of SST-1M telescopes is currently being commissioned at the Ondrejov Observatory in the Czech Republic, and the telescope capabilities for mono and stereo observations are being tested in better astronomical conditions. The final location for the telescopes will be decided based on these tests. In this contribution, we present a data analysis pipeline called sst1mpipe, and the performance of the telescopes when working independently and in a stereo regime.

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